This invention relates generally to magnetic resonance imaging and more particularly the invention relates to stabilization of a selective slice profile for segmented k-space imaging.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.O, of the precession of the nuclei is the product of the magnetic field B.sub.O, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.O =B.sub.O..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =Z.G.sub.z, on the static uniform field, B.sub.O, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclear spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
A k-space interpretation of nuclei excitation is given by Pauly, Nishimura, and Macovski in "A k-space Analysis of Small-Tip-Angle Excitation," Journal of Magnetic Resonance 81, 43-56 (1989). They showed that multi-dimensional selective excitation in the presence of time-varying gradients can be analyzed using Fourier transform theory. Using this interpretation, they designed and implemented selective excitation pulses that are selective in two spatial dimensions. Based on a small-tip-angle approximation, selective excitation is interpreted as a weighted trajectory through k-space. The slice profile is the Fourier transform of this weighted trajectory.
Segmented k-space imaging sequences make use of a short burst of selective pulses to acquire multiple phase encodes every cycle. As there is only partial recovery of longitudinal magnetization between pulses, an amplitude modulation of the phase encodes will be incurred if the pulses are not designed to compensate.
To reduce blurring and ghosting caused by this amplitude modulation, an increasing tip-angle sequence can be used to stabilize signal level in the middle of the slice. However, significant variations in the signal profile will still occur at the edges. The present invention is directed to a method for stabilizing the entire signal profile.